The composition of the mammalian microbiota can change in response to metabolic, pharmacologic, or inflammatory cues and these alterations have the potential to initiate or reinforce inflammatory disease. Multiple studies have shown that the host immune system plays a prominent role in regulating the structure of the intestinal microbiota but most of the detailed mechanisms come from studies on Segmented filamentous bacteria, Clostridium, or highly attenuated Salmonella strains. This exploratory application will use newly developed commensal and probiotic strains of E. coli to visualize E. coli-specific CD4 T cell responses in vivo. Our application specifically proposes to, (i) anatomically define E. coli-specific CD4 T cell activation and explore the role of CD103- dendritic cells in driving mucosal and systemic commensal-specific responses and, (ii) evaluate the functional outcome of CD4 T cell responses to commensal E. coli. Greater understanding of the basic immune response to intestinal E. coli should provide foundational knowledge for understanding tolerance and inflammatory responses in the intestine.